The making of steel or other metals typically involves the introduction of gases into the vessel holding the molten metal. The gas is typically introduced into the vessel via a refractory gas purging device also known as purging cones, porous plugs, or gas purging elements. The refractory gas purging device may be mounted in the bottom or side of the vessel. Prior art refractory gas purging devices fall into two main categories.
The first category comprises conical shaped members or plugs formed of a porous refractory member through which the purging gas is passed. The plug utilizes the porosity of the material for carrying the gas through the device. The porosity is a result of the proper mix design and the proper heat exposure of the refractory mass during the manufacture of the refractory component of the cone. The typical diameters of the pores through which the gas passes are between 0.01 mm to 0.08 mm, which is small enough to prevent the infiltration of molten metal through the plug. These gas purging devices offer a high degree of operational safety, however, the porous refractory material has a limiting effect on the wear resistance of the gas purging device.
The second category comprises directional porosity plugs made from dense refractory materials with formed gas directional channels. Such designs are disclosed in U.S. Pat. No. 4,905,971 and DE-OS 32 46 937. The gas channels are created in-situ, with use of fluidizing material or with embedded hollow tubes. The channels can also be mechanically formed on the outside surface of an internal ceramic element or the inside surface of an external ceramic element. The channels also can be designed as a gap between an external metal can surrounding the plug and the internal dense refractory material. The profiles of the channels, which are also called directional porosity, can be in the form of capillaries, capillary networks, gaps, and slits. The directional porosity extends throughout the entire length of the cone. These directional porosity plugs, in which the dimensions of the gas passages are a magnitude larger than those of the permeable plugs in the first category, offer relatively low operational safety. The liquid metal can easily infiltrate the gas channels eventually causing the plug to act as a sink causing leaks of molten metal from the vessel.
There have been improvements to refractory gas purging devices concerning safety and wear indication. U.S. Pat. No. 4,396,179 and 4,725,047 disclose a device for introducing gas into molten metal having external safety elements to prevent the leakage of molten metal through the device. The disadvantage of these devices is the high cost of manufacturing the device.
U.S. Pat. No. 5,202,079 and German DE-PS 31 42 989 disclose gas purging devices having internal optical wear indicators. The optical wear indicators warn the end of the useful life of the device by changes in the emissivities of the refractory components, or by the change of the shape of the gas passages through the device. The disadvantages are that the wear indication can only be detected during the examination of the empty metallurgical vessel and the shape change of the wear indicator in directional porosity plugs is difficult to recognize during the inspection.
It is desirable to have a refractory gas purging device which has improved safety, wear indication, and performance and which do not suffer from the problems of the refractory gas purging device of the prior art.